AlkynesAlkynes

Hydrocarbons with a carbon–carbon triple bond are alkynes.

Noncyclic alkynes have the molecular formula CnH2n-2.

Acetylene (HC≡ CH) is the simplest alkyne.

Compounds with the triple bond at the end of a carbon

chain (RC≡ CH) are monosubstituted, or terminal, alkynes.

Disubstituted alkynes (RC≡ CR′) have internal triple bonds.

AlkynesAlkynes

In the late 19th century calcium carbide was formed by heating coke and limestone.

Sources of AlkynesSources of Alkynes

The calcium carbide was hydrolysed to form acetylene.

One alternative synthesis is the dehydration of ethylene.

The endothermic reaction favors acetylene at high temperature.

Sources of AlkynesSources of Alkynes

Enzymes known as acetylenases catalyze this reactionin nature.

Some bacteria produce dynemicin A which has been shown to cleave DNA.

Naturally Occurring AlkynesNaturally Occurring Alkynes

Some fatty acids contain an alkyne, e.g. stearolic acid.

Diacetylene has been identified asa component of the atmospheresof Uranus, Neptune and Pluto.

Replace the suffix –ane for the corresponding alkane with –yne.

Naming AlkynesNaming Alkynes

Compounds that contain both an alkene and an alkyneare numbered to give the first multiple bond the lowest number and are named as an –enynes.

Naming AlkynesNaming Alkynes

hept-1-en-5-yne

Alkynes resemble alkanes and alkenes in physical properties.

Low density and low water solubility.

Boiling points similar to the corresponding alkane.

Physical Properties of AlkynesPhysical Properties of Alkynes

Acetylene is linear.

Structure of AlkynesStructure of Alkynes

Cyclononane, the smallest stable cycloalkyne is strainedas the C-C≡ C-C unit is clearly bent.

The C≡ C has two -bonds and one -bond. The -bond is formed by overlap of sp orbitals. The -bonds areformed by overlap of p orbitals. The -bonds are orthogonal to each other – shown in (b) and (c).

Bonding in AlkynesBonding in Alkynes

The electrostatic potential maps shows the high electron density (red) associated with the -bonds.

Bonding in AlkynesBonding in Alkynes

Bonding in Alkanes, Alkenes and AlkynesBonding in Alkanes, Alkenes and Alkynes

Bonding in Alkanes, Alkenes and AlkynesBonding in Alkanes, Alkenes and Alkynes

For the series ethane → ethylene → acetylene the general trends are:

1. The geometry at carbon changes from tetrahedral → trigonal planar → linear.

2. The C-C and C-H bonds become shorter and stronger.

3. The acidity of the C-H bonds increases.

C-H bond lengths are shortest for hydrogens bonded toa sp C. The sp orbital has higher s character than sp2 orbitals so it is closer to the carbon atom.

C-H Bond LengthsC-H Bond Lengths

C-H bonds of alkanes, alkenes and alkynes are very weak acids. The pKa of methane is estimated to be 60.

Acidity of Alkanes, Alkenes and AlkynesAcidity of Alkanes, Alkenes and Alkynes

Because it is derived from a very weak acid the conjugate base, a carbanion, is a very strong base.

The effective electronegativity of carbon increases with increasing s-character (sp3>sp2>sp) therefore the pKa decreases in the same order.

Acidity TrendAcidity Trend

After deprotonation the acetylide electron pair is in a sp orbital.

Acidity of AlkynesAcidity of Alkynes

Terminal alkynes RC≡ CH are similar to acetylene in acidity.

In order to form the acetylide anion the base must bestrong enough. The conjugate acid of the base should have pKa greater than the pKa of the alkyne.

Hydroxide is too weak and the equilibrium lies to the left.

Acid-Base Reactions of AlkynesAcid-Base Reactions of Alkynes

Amide is strong enough and the equilibrium lies to the right.

Preparation of Alkynes by Alkylation Preparation of Alkynes by Alkylation of Acetylene and Terminal Alkynesof Acetylene and Terminal Alkynes

Acetylide anions react with methyl and primary alkyl halides to form substituted alkynes.

Acetylide Anions as NucleophilesAcetylide Anions as Nucleophiles

This is an SN2 type of reaction.

Acetylide Anions as NucleophilesAcetylide Anions as Nucleophiles

Examples include:

The reagents are listed above and below the reaction arrow in the order they are used. So, in the first reaction,acetylene is first deprotonated with sodium amide in ammonia and then reacted with ethyl bromide.

Acetylide anions are very basic so they act as bases if the alkyl halide is secondary or tertiary.

Elimination Reactions with Acetylide AnionsElimination Reactions with Acetylide Anions

Alkynes can be prepared by double dehydrohalogenation of either geminal dihalides or vicinal dihalides.

Preparation of Alkynes by EliminationPreparation of Alkynes by Elimination

Three equivalents of base are required if the alkyne is terminal since a terminal alkyne is deprotonated by the base as soon as it is formed. Protonation of the acetylide anion is then required as a second step.

Preparation of Alkynes by EliminationPreparation of Alkynes by Elimination

Vicinal dibromides are formed on bromination of an alkene so this gives us a way to transform an alkeneinto an alkyne.

From Alkenes to AlkynesFrom Alkenes to Alkynes

Reactions of alkynes are similar to reaction of alkenes:hydrogenation, hydration, halogenation, ozonolysis.

Reactions of AlkynesReactions of Alkynes

Alkynes are reduced to alkanes using hydrogen and a transition metal catalyst.

Hydrogenation of AlkynesHydrogenation of Alkynes

For example:

The heat of hydrogenation of 1- and 2-butyne can be used to determine the relative stability of an internal and a terminal alkyne (both yield butane as product).

Relative Stability of AlkynesRelative Stability of Alkynes

Less heat is released by 2-butyne so that is the more stable isomer.

Metal catalyzed hydrogenation is a syn addition process.

Stereochemistry of HydrogenationStereochemistry of Hydrogenation

Lindlars catalyst is a partially deactivated catalyst that

was developed for partial hydrogenation of alkynes to alkenes.

Hydrogenation with Lindlars CatalystHydrogenation with Lindlars Catalyst

Disubstituted alkynes yield cis-alkenes.

Group I metals in liquid ammonia reduce alkynes to trans alkenes exclusively.

Metal-Ammonia Reduction of AlkynesMetal-Ammonia Reduction of Alkynes

The key intermediate is the vinyl radical which prefers theless hindered trans conformation.

Step 1. Electron transfer.

Mechanism of Metal-Ammonia ReductionMechanism of Metal-Ammonia Reduction

Sodium atoms dissolved in liquid ammonia dissociate into sodium ions and electrons, both solvated by ammonia. The solvated electrons are represented in e–(am).

Step 2. Proton transfer.

Step 3. Electron transfer.

Mechanism of Metal-Ammonia ReductionMechanism of Metal-Ammonia Reduction

Step 4. Proton transfer.

Hydrogen halides add to alkynes to form alkenyl halides.

Addition of Hydrogen HalidesAddition of Hydrogen Halides

The reaction follows Markovkovs Rule (the proton adds to the carbon that initially has the most hydrogens).

Markovnikov addition suggests formation of a vinyl cation:

Mechanism of Addition to AlkynesMechanism of Addition to Alkynes

Kinetics studies however suggest the involvement of two equivalents of HX: rate = k[alkyne][HX]2

A possible mechanism is:

Mechanism of Addition to AlkynesMechanism of Addition to Alkynes

Excess HX leads to geminal dihalides.

Addition of HX to AlkynesAddition of HX to Alkynes

Since each addition of HX follows Markovnikov’s ruleboth protons are added to the same carbon.

Anti-Markovnikov addition of HBr is performed with peroxide initiation.

Free Radical Addition of HBrFree Radical Addition of HBr

Hydration of alkynes yields a special alcohol – an enol. The enol has an –OH attached to an alkene carbon.

Hydration of AlkynesHydration of Alkynes

The enol rapidly isomerizes to the keto form. This equilibration is keto-enol tautomerism.

Tautomers are constitutional isomers that equilibrate bymigration of an atom or group.

Step 1. Protonation.

Mechanism of Conversion Mechanism of Conversion of an Enol to a Ketoneof an Enol to a Ketone

Step 2. Deprotonation.

Resonance forms:

In general the keto form is more stable so it will be drawnas the product of a hydration reaction.

Hydration of AlkynesHydration of Alkynes

The reaction follows Markovnikov’s rule so terminal alkynes yield methyl ketones not aldehydes.

Two molecules of halogen react with one alkyne to yielda tetrahaloalkane.

Addition of Halogens to AlkynesAddition of Halogens to Alkynes

An anti dihaloalkene can be isolated if exactly one equivalent of halogen is used.

Addition of Halogens to AlkynesAddition of Halogens to Alkynes

We can write simple reactions to produce these usefulmonomers from acetylene.

What Could be Made From Acetylene but Isn’tWhat Could be Made From Acetylene but Isn’t

In reality they are actually made from ethylene because itis so much cheaper and easier to prepare than acetylene!

Ozonolysis also cleaves alkynes. The alkyne carbons are tranformed into carboxylic acid carbons. The end carbon of a terminal alkyne becomes carbonic acid which dissociatesto CO2 and H2O.

Ozonolysis of AlkynesOzonolysis of Alkynes

Alkynes in Synthesis and Alkynes in Synthesis and RetrosynthesisRetrosynthesis

Applications of Alkynes in SynthesisApplications of Alkynes in Synthesis

Example: Outline a synthesis of 1,2-epoxybutane using ethyl bromide and acetylene as sources for all the carbon atoms.

The epoxide may be formed from the corresponding alkene:

The alkene could be made from the corresponding alkyne which could itself be prepared from an acetylide anion:

Applications of Alkynes in SynthesisApplications of Alkynes in Synthesis

Now list the reactions in the “forward” direction starting from acetylene.